Television transmitter



Oct. 27, 1942.

A. v. BEDFQRD TELEVISION TRANSMITTER Filed Oct. 24, 1940 3 Sheets-Sheetl 2 F/FfQUE/VCYl/V C(ttorneg Oct. 27, 1942.

A. v. BEDFORD TELEVISION TRANSMITTER 3 Sheets-Sheet 2 Filed Oct. 24,1940 Bnventor 0rd .lldwZ (Ittomeg Oct. 27, 1942. A. v. BEDFORDTELEVISION TRANSMITTER 3 Sheets-Sheet 3 Filed Oct. 24, 1940 3noentor 0rdMira v attorney Patented Oct. 27, 1942 TELEVISION TRAN SMITTER Aida V.Bedford, Collingswood, N. 1., assignor to Radio Corporation of America,a corporation of Delaware Application October 24, 1940, Serial No.362,547

'5 Claims. (Cl. 178-72) lem is simply that of obtaining a useablepicturesignal under the lighting conditions available.

At one time it was the practice to make the resistance of the outputresistor of a cathode ray pick-up tube low enough to give-the outputcircuit of this tube a fiat frequency-response characteristic over theentire picture frequency range.

More recently the output resistor has been given a much higher valuethan indicated above whereby the picture signal components are held upto a good signal strength for the lower frequency components (up toabout 100 kilocycles, for example). Generally the output resistor has aresistance of from 50,000 ohms to 200,000 ohms and the stray capacitancethereacross is from to micro-microfarads. The response characteristictherefore is not fiat and falls oil for the higher frequencies, thisfalling oil of the characteristic being compensated for in a laterstage. The circuit just described is usually referred to as the highpeaker circuit and is covered by my Patent 2,151,072 issued March 21,1939, and assigned to the Radio Corporation of America.

An object of the present invention is to further improve thesignal-to-noise ratio of a picture signal by holding up or accentuatingthe picture signal amplitude in the region of the higher frequencycomponents and later compensating for this accentuation.

Stated more broadly, an object of the invention is to increase theuseable signal output of a picture transmitter tube. p

In practicing my invention an inductance coil is connected between theoutput electrode of the cathode ray transmitter tube and the controlgrid of the first amplifier whereby the output capacity of thetransmitter tube and the input capacity of the amplifier are separatedby the coil. This forms a low pass filter section. If conventionalfilter practice were followed, this filter section would be terminatedin its surge impedance to. prevent reflections. However, if this weredone, the transmitter tube would work into an impedance too low forobtaining satisfactory signaL-to-noise ratio, the impedance under theseconditions being of the order of 3000 ohms, for example, where theabove-mentioned capacities are about 10 micro-microiarads each and wherethe inductance coil is adjusted for a cut ofi frequencyof about 4.5megacycles. Because 01' this, prior to my invention the separation oftube capacities in the pick-up tube output circuit by means of aninductance coil was considered undesirable.

In the present invention I discard the conventional theory that majorreflections at the end of a filter should not be allowed. it Instead ofterminating the filter section in its surge impedance of 3000 ohms, forexample, it is terminated'in a very high resistance such its-100,000ohms. This results in a tremendous peak in the picture signal in acertain region of the higher frequency components, the location of thispeak depending upon the inductance value of the coil.

After a suitable amount of amplification, the picture signal is passedthrough a compensating circuit which has the proper characteristic tomake 'the over-all frequency response characteristic ofthe pick-up tubeand associated amplifier and compensating circuit substantially flat.This means, or course, that the compensating circuit is so designed thatthe picture signal components in the region of the above-mentioned peakare amplified less than the picture signal components of less amplitude.The result is that the noise, also, in the region of picture signal peakis amplified less whereby the overall signal to noise ratio is improved.The compensating circuit, in addition to providing amplitude correction,corrects for undesirable phase shift which is introduced by the filtersection.-

The invention will be better understood from the following-descriptiontaken in connection with the accompanying drawings in which:

Figure 1 is a circuitdiagram of one embodiment of my invention;

Figure 2 is a group of curves showing 'frequency responsecharacteristics which are referred to in' explaining the circuit of Fig.1,

Figure 3 is a curve showing the square-wave response ofthe circuit ofFig. 1 with the correcting network omitted,

Figure 3a isra curve showing the square-wave response of the circuit ofFig. 1 where the correcting network is included,

Figure 4 illustrates one form of correcting network that may be utilizedin the circuit of Fig. 1, Figure 5 is a'circuit diagram illustrating anembodiment of my invention in which the corvarious ways.

rection or compensation is obtained in successive amplifier stages bydifferent networks,

Figure 6 is a group of curves which are referred to in explaining theoperation of the network of Fig. 4, and Figures '7, 8 and 9 are circuitdiagrams illustrating other correcting circuits which may be employed inpracticing my invention, Figure 9 being the preferred circuit.

Referring to Fig. 1 the transmitter circuit comprises a cathode raytransmitter or pick-up tube in consisting of an evacuated envelopehaving therein an electron gun II for producing a beam of electrons, asecond anode I2 and a mosaic ll of light sensitive elements. In thisparticular type of pick-up tube a light image of the picture to betransmitted is projected upon the mosaic l3 by means of a suitable lenssystem, not shown.

This causes an electrical image of the picture to be formed on themosaic whereby picture signals representative of the conditions of lightand shade in the picture are reproduced as the mosaic is scanned by theelectron beam. The beam is caused to scan the mosaic by means ofdeflecting coils or plates, not shown. It will be understood that theinvention is not limited to the specific type of pick-up tube Hi, thistype of tube being selected merel for the purpose of giving a specificexample. Also,the invention may be applied to signal output devicesother than picture signal producing devices.

As stated in my above mentioned high peaker patent, and as well known inthe art at the present time, the mosaic l3 may be constructed in In allcases, however, it will be found that there is a substantial amount ofcapacity between the output electrode of the pickup tube and ground, andthat this is one of the limiting factors in obtaining a picture signaloutput of substantial amplitude for the higher frequency components ofthe picture signal. As explained in my above-mentioned patent thisdifflculty can be overcome to a certain extent by making the outputresistance of the cathode-ray tube much higher than the value requiredfor a flat over-all response and compensating at a later point in thecircuit for the resulting falling off of the signal amplitude at thehigher frequency components.

In accordance with the present invention, in addition to giving theoutput resistor, shown at IS, a high resistance value as taught in myhigh peaker patent, I connect an inductance c011 L between the outputterminal of the cathode-ray tube 10 and the control grid ll of the firstamplifier tube l8 whereby the output capacity of the cathode-ray tubeindicated at Cl and the input capacity of the first amplifier tubeindicated at C2 are separated. It will be evident that now instead ofthe sum of the capacities Cl and C2 appearing betweenthe output terminalof the cathode-ray tube and ground, the cathode-ray tube feeds into asection of a low pass filter In terminating this filter section Cl-LC2,

the filter section is terminated in a resistance of 100,000 or 200,000ohms, for example.

It will be understood that the inductance value of the coil L willdepend upon the other circuit constants and upon the desired location ofthe resonant peak in the curve Bl of Fig. 2. As an example, however, theinductance of coil L may be of the order of 300 microhenrys.

As a result of the high resistance filter termination, the frequencyresponse curve, taken at 'the control grid 11 of the first amplifiertube, is

as represented by the curve Bl (Fig. 2). It will be seen that this curvehas a very high peak in i the region of the high frequency components ofthe picture signal as a result of the high resistance termination of thefilter section. It will be noted that curve Bl also has a largeamplitude in the region of the lower frequency components of the picturesignal as a result of the benefit derived from the high resistance ofthe output resistor l6 as described in my high peaker" I patent.

however, conventional filter theory is not followed since filter theorywould require that the output resistor [6 (which terminates the filtersection) have a resistance very much lower than the resistance actuallyemployed. As previously indicated, filter theory would require that thefilter sectionbe terminated in a resistance of about 3000 ohms whereasin practicing my invention,

Perhaps it should be mentioned that the frequency response curves in thepresent application and those shown in my high peaker" Patent 2,151,072are drawn to scales which are so unlike that at first glancecorresponding curves in the two cases do not look alike. In the highpeaker" patent, the curves are drawn up to one megacycle only and theyare drawn to a logarithmic scale which stretches out" the low frequencyend of the scale. 0n the other hand the frequency scale for the curvesof Fig. 2 in the present application is linear and goes out to 5megacycles.

In actual practice it usually is desirable to accentuate the very lowfrequency components, components of the order of 60 or cycles persecond, for example, by means of a condenser 2| connected in series withthe output resistor l6 and shunted by a resistor 22. The desirednegative bias for the amplifier tube It! may be provided by a biasingbattery 23. In the illustration of Fig. 1, the condenser 2| and resistor22 are shown short circuited by means of a switch 24 to make themineffective. The curves in Fig. 2 are drawn for the condition ofoperation where the switch 24 is closed.

. The use of the circuit 21-22 for accentuating the very low frequencycomponents is described in my Patent 2,200,073 issued May '7, 1940. Hereagain, the frequency scale for the curves in the patent is logarithmicwhereby the curve in the region of the lower frequencies is stretchedout. Since this accentuation of the signal occurs only in the generalregion of 100 cycles it is apparent that it could not very well be shownin Fig. 2 where the frequency scale is linear and covers 5 megacycles.

Still referring to Fig. l, the amplifier tube l8 preferably is providedwith the usual inductance coil 20 for holding up the high frequencyresponse. The output of this amplifier is passed through one or moreamplifier stages, such as the one including amplifier tube 21, andsupplied to a suitable correcting or compensating network indicated at28. As will be explained hereinafter, the correcting network 28 may bedesigned in accordance with the teachings of the copending applicationSerial No. 266,756, filed April 8, 1939, as a joint invention ofBlumlein, Kallmann and Percival and assigned'to Radio Corporation ofAmerica.

The operation of the circuit embodying my invention will now be morefully explained with als, there is illustrated reference to the curvesin2. .I'hecurve Al, which at the lower frequenciescoincides with i hecurve Bl, represents the frequency response -characteristicor thecircuitho! Fig. 1 with the coil L Omitted. It will be; ev dent that theuse bf the high resistance outputr istor; II has held up the picturesignal outputin the r gi n. below one megacycle. Following theteachin'gsof my ,high peaker fPatent 2,151,072 a flat over all replifler tube isthat shown at B], there being a highpeak in this curve in the region ofthe higher frequency components of the picture signal, thispeaking beinglocated between [and 5 megacycles in the particular example illustrated.The correcting or compensating network 28 is designed to have thefrequency response characteristic shown at B2. The frequency responsecharacteristic B2 is made such that the overall frequency response isflat. It will be noted that the hat over-all frequency response curve(not shown) is obtained by multiplying the ordinates of curves BI and B2together rather than by adding them together since the over-all gain oftwo stages in cascade is obtained by "multiplying,

rather than by adding the gains of the individual stages, i

The characteristic ofthe circuit of Fig. 1 with and without thecorrecting network 28 is further illustrated by Figs. 3 and 3a. Fig. 3shows thesquare-wave responseof the circuit with the coil L but without,the correcting network, while Fig. 3a shows the square-wave responseofthecircuit after the undesirable transient response introduced by thecoil L has been compensated for by means of the correcting network, Asillustrated in Fig. 3a, the square-wave response of the correctedcircuit preferably has a small amount, of "overswing, indicated, at X,becauseexperience has shown that this is desirable for best picturequality. t

In Fig. 4,whe re parts corresponding to those in Fig. 1 are indicated bylike reference numernetwork that may be employed in practicing myinvention, this being one of the networks deone form of correctingscribed and claimed in the above-mentioned Blumlein et al. application.It comprises a delay network of conventional design consisting of aplurality of sections each having series inductance indicated at 3| andshunt capacitance indicated at 32, the network being suitably terminatedby a resistor 33. Signal is taken off this delay network at suitablepoints through resistors 34 and combined in an output resistor 36 ofcomparatively low impedance. Signal is taken on the delay network atother points through resistors 31 and, after being reversed in phase byan amplifier tube 38, is combined with the rest of the signal in theoutput resistor 35, By properly selecting the delay, amplitude and phasereversal as taught in the above-mentioned Blumlein et a1. applicationthis correcting network 28 may be given any desired characteristic; Inthe present instance the network 28, of course, is designed to have thefrequency response characteristic shown by the curve 32 in Fig. 2 inorder to correct for the frequency response characteristic Bl oi thepreceding apparatus.

of course, that the speillustrated in Fig.4 is purely for the purpose ofillustrating the g ener'alchar acter of the Blumlein et a1; type ofnetwork and that it, might number of delay network sections and adifierent It will be understood,

numbcrof points at whichthe signal is tapped on. and/or reversedlnphase. a

The principal of operation of the correcting network shown in, Fig. 4 isillustrated by the curvesin Fig. 6, this figure corresponding to one ofthe figures in the-Blumlein ,et a1. application. The procedure indesigning the correcting network is as follows: First, thedesiredfrequency response characteristic {or the network is determinedthis being re resented by thecurve B2 of Fig. 1 in the example underconsideration. Next the square wave or transient response refi networkhaving this frequency. response characteristic is determined bycalculation. For the purpose of illustration it will be assumed thatthis transient response is. represented by the curve I in Fig. 6. It isthen possible in a network such asshown in Fig. 4 to select the pointsat which the signal is delayed the proper amount and to select theproper si nal amplitude {to build up the desired curve asshown by thesmall stepped curves 40a, 40b, 40c and 40d in Fig. 6. It-will be evidentthat where there is a dip in the curve 40 as at y, it is necessary toreverse the phase of some of the delayed signal. 1

The great improvement that may be obtained in the signal-to-noise ratioby practicing my invention will be seen more clearly from the fol-.

ture signal output for the case where the inductance coil L is omittedas for the case where it is included in the circuit. Sincemost of thenoise in the system generally originates in the first amplifier tube 18(assuming the case where the,

output resistor l8 has large resistance), the'relative noise obtainedwith my inventionas compared with that obtained without my invention isshown bya comparison of the ordinates of curve B2 and A2, vention causesa substantial reduction innoise in the picture signal since the curve B2is much lower in amplitude than the curve A2 over a large portion of theupper frequency range.

If the coil L were employed and the resulting filter section terminatedin its surge impedance, the relative response at the grid of amplifiertube l8 would be so low that the high subsequent am plificationrepresented by curve A3, for example, would be required over the entirefrequency band of signal components. The disadvantage with respect tosignal-to-noise ratio is obvious.

The position of the peak of the curve Bl in the high frequency regionmay be shifted either toward the high end or toward the low end of thefrequency band by changing the value of the inductance coil L. It hasbeen found that, in some cases, at least, it may be preferable to havethis peak located at a slightly lower frequency than shown in Fig. 2 ifmaximum noise reduction is to be obtained. 7 i

It is usually desirable to operate the circuits of Fig. 1 with theswitch 24 open whereby the previously described accentuation of the verylow frequency components is obtained. It will be understood that when sooperated there is provided at some later point in the circuit a combedesirableto have a'difl'erent respectively. Obviously, my inplementaryattenuating-means such as a In the circuit of Fig. 5 there is providedin the 4 output circuit of the amplifier tube 21 a high peaker circuitR1-C1-Rl of the character described in my high peaker Patent No.2,151,072. In this output circuit, an operating voltage preferably issupplied to the plate of the amplifier tube 21 through a plate resistor4| and through the usual inductance coil 42 provided for holding up thehigh frequency response of the amplifier. The inductance coil 42 may beomitted if the plate resistor 4| is given a resistance low enough tomake the amplifier characteristic substantially fiat, but good amplifierpractice usually indicates the use of the coil 42. The condenser 43functions-merely as a coupling condenser and for this purpose shouldhave fairly large capacity. In

- the high peaker circuit itself, the resistor R5 has a resistance whichis low compared with thatof the resistor R1 whereby the percentageof-the total signal voltage for a given frequency component appearingacross the resistor R5 becomes greater for the higher frequencies.

The frequency response characteristic of the high peaker networkR1-C1-R5 itself is shown essentially by the curve A2 of Fig. 2.

The desired over-all frequency response characteristic represented bythe curve B2 is'obtained by includingin the next amplifier stage anetwork of the type previously described in connection with Figs. 1 and4. This correcting network comprises a delay network consisting of aplurality of sections each includinga series inductance 46 and a shuntcapacitance 41, the delay network being suitably terminated inaccordance with conventional practice by means of a resistor 4!. Aspreviously described, the picture signal is taken oil the delay networkat suitable points and supplied through resistors 40 to an amplifiertube ii and through resistors 52 and a phase-reversing tube 53 to anamplifier tube 54 having a plate circuit common to the amplifier tube 5|whereby all the picture signals are added and appear across the outputresistor 55.

By referring to Fig. 2, it will be seen that in the lower frequencyregion of the picture signals, the curve B2 coincides with the curve A2.It is evident that, in this region, the correction is proponents, thecorrection represented by the curve B2 is provided by a combination ofthe characteristics of the circuit'R1-C1-R5 and of the correctingnetwork in the following stage.

In Figs. 7 and 8, there are shown other circuits for obtaining thedesired frequency response characteristic B2, these circuitsalsocomprising a high peaker amplifier stage and another stage incascade therewith to compensate for the high resonant peak of the curveBl. In these figures the parts corresponding to like parts in Fig. 1 areindicated by the same reference numerals.

I Referring to Fig. '1, the high peaker stage comprises the amplifiertube 21 which has an opcrating voltage applied to its plate through aninductance coil ti and a plate resistor 02. this resistor having muchlower resistancethan would beemployedinan amplifier stage designedtohavea fiat overall "frequency response characteristic. Therefore, this stageitself has the risingv characteristic A2 of Fig. 2. The design of such ahigh peaker" stage is described in my abovementioned Patent No.2,151,072. In a following amplifier stage 62, there is connected acrossthe plate circuit of the amplifier tube a series-resonant circuitconsisting of an inductance coil I4,

a condenser 66, and a resistor 81, this circuit being tuned to beseries-resonant at the frequency at which the high resonant peak appearsin the curve Bl. By varying the resistance of the series resistor 61,the amount of damping in the circuit may be varied for the purpose ofshaplnt the frequency response characteristic. The two stages incombination have the characteristic B2 of Fig. 2.

In Fig. 8, the high peaker" circuit R|C0Rl is the same as that shown inFig. 3. In a following stage,= compensation for the high resonant peakof the curve BI is obtained by employing in the plate circuit of anamplifier tube 00 a 'correcting network Cl0Ll0Rl0-Ril which is similarto the high peaker circuit of the preceding stage except that itincludes the inductance coil Ll0. This inductance coil Llll and thecondenser C10 are tuned to form a circuit which is parallel resonant atthe frequency of the high resonant peak in the curve Bl, whereby theamount of signal flowing through the resistor R, at this frequency isvery small, as indicated by the dip in the curve B2.

In Fig. 9, where parts corresponding to those in Fig. 1 are indicated bythe same reference characters, there is-illustrated a preferred circuitwhich may be substituted for the correcting network 28 indicated in Fig.1, this circuit being preferred because of its simplicity and ease ofadjustment. It is substantially the same as the high peaker circuitillustrated in Fig. 5, with the addition thereto of a series-resonantcircuit consisting of an inductance coil Ll2, a condenser CH, and aresistor R12, connected in series with each other and connected acrossthe output resistor Bit of the high peaker"-network. The saidseries-resonant circuit is tuned to series resonance at the frequency ofthe high resonant peak in the curve Bl. V

As described in connection with Fig. 3, the plate of the amplifier tube21 has an operating voltage applied thereto through a plate resistor 1|and an inductance coil 12 provided for holding up the high frequencyresponse of the amplifier tube 21. The condenser 13 functions simply'asa coupling condenser.

The specific circuitshown in Fig. 9 is being claimed in copendingapplication Serial No. 362,- 494, filed on the same day as the presentapplication, in the name of Gordon F. Fredendall, andance coilconnecting said output terminal to said- I 2,399,975 control electrodewhereby said coil and said capacities form a filter section, atermination for said filter section comprising a resistor having suchhigh resistance that said section is terminated in an impedance manytimes its surge im- Dedance whereby the signal output appearing at saidcontrol electrode has a resonant peak in the region of its higherfrequency components, and

-a resistor having such high resistance that said section is terminatedin an impedance many times its surge impedance whereby the picturesignal output appearing at said control electrode has a resonant peak inthe region of its higher frequency components, and means connected incascade with said amplifier tube to correct for amplitude and phasedistortion introduced by said filter section.

3. The invention according to claim 2 wherein said filter terminatingresistor functions as a grid leak for said amplifier tube and whereinsaid resistor is the only direct current connection between the outputterminal of said picture converting device and ground.

4. Picture transmitting apparatus comprising an electric dischargedevice for converting pictures into electrical signals, said devicehaving an output terminal which has unavoidable capacity to ground, anamplifier tube having a control electrode which has unavoidable capacityto ground, an inductancecoil connecting said output terminal to saidcontrol electrode whereby said coil and said capacities form a low passfilter section, a termination for said filter section comprising aresistor having such high resistance that said section is terminated inan impedance many times its surge impedance whereby, at the controlelectrode of said amplifier tube, the frequency response is held up forthe lower frequency components of the picture signal and whereby thereis produced a resonant peak in the frequency response in the region ofthe high frequency components of the picture signal, and a compensatingcircuit connected in cascade with said amplifier tube and designed tohave a frequency response characteristic substantially complementary tothe frequency response characteristic of the apparatus as measured atsaid control electrode.

5. Picture transmitting apparatus comprising a cathode ray tube havingtherein a mosaic of capacity elements positioned to be scanned by thecathode ray whereby picture signals may be caused to appearat an outputterminal of said cathode ray tube, said cathode ray being designed toproduce picture signals having frequency components up to at least fourmegacycles, an amplifier tube having an input electrode, an inductancecoil connecting said output terminal to said input electrode whereby a.low pass filter section is formed as a result of tube capacity toground, a high resistance termination for said filter section, saidtermination having an impedance many times the surge impedance of saidfilter section, and means connected in cascade with said amplifier tubeto compensate for the resulting falling off of the frequency responsecharacteristic at the high picture frequency component as measured atsaid input electrode and to compensate for the resulting resonantpeakappearing in said characteristic in the region of the higherfrequency signal components as measured at said input electrode.

ALDA V. BEDFORD.

